Three sexually dimorphic cell groups in the forebrain of the rat-the central part of the medial preoptic nucleus, the encapsulated part of the bed nucleus of the stria terminalis, and the posterodorsal part of the medial nucleus of the amygdala-are larger in males, contain a high density of gonadal-steroid-concentrating cells, and are thought to play important roles in the control of reproductive behavior and physiology. Since each of these regions contains a large number of cholecystokinin-immunoreactive cells, we used an indirect immunohistochemical method to examine the possibility that levels of this peptide are modulated by circulating gonadal steroids in adult male rats. Rats were castrated at 60 days of age, and one group each was pretreated with colchicine and then killed 3, 7, and 14 days after gonadectomy. Castration clearly decreased CCK immunoreactivity within cells of each region, with the most dramatic effects occurring 7 and 14 days after gonadectomy, and these effects were reversed by treatment with testosterone over a 14-day period. The results suggest that CCK levels within individual cells in each of the interconnected sexually dimorphic nuclei examined here are regulated by circulating gonadal steroids and may be related to the hormonal modulation of reproductive functions thought to be mediated by these cell groups.An important role for gonadal steroid hormones in the control of reproductive behavior is well-established, although mechanisms underlying their influence on neural circuits mediating such functions remain unclear. Gonadal steroids have been shown to markedly affect biochemical indices of neurotransmitter function (see ref. 1 for review), and steroid-induced changes in the levels of putative neurotransmitters within individual neurons have been demonstrated immunohistochemically. For example, 15 weeks after gonadectomy, the density of vasopressin-immunoreactive fibers in the lateral septum is markedly reduced in the adult male rat, and vasopressin-stained cell bodies are no longer demonstrable in the bed nucleus of the stria terminalis and medial nucleus of the amygdala (2). Similarly, the density of substance Pimmunoreactive fibers decreases within 14 days after gonadectomy (3). In contrast, the distribution of serotoninimmunoreactive fibers in the medial preoptic nucleus (MPN) and the density of tyrosine hydroxylase-immunoreactive (presumably dopaminergic) cells in the anteroventral periventricular nucleus do not appear to be sensitive to the influences of circulating gonadal steroids in the adult but can be dramatically altered by manipulation of the perinatal steroid environment (4, 5).The sexual differentiation of neural structures in the mammalian brain has been documented for a number of cytological features of individual neurons, the synaptology of specific brain regions, and the volume and cytoarchitecture of several nuclei (see refs. 6 and 7 for reviews). Three such nuclei, the medial preoptic nucleus (especially its central subdivision, MPNc; ref. 8), the encaps...